Bilinguals know virtually twice as many words as monolinguals. They seem to effortlessly use the right word in the right context, and can even switch back and forth between languages with little obvious cost. Nonetheless, evidence suggests that maintaining more than one language in a single cognitive system introduces processing differences, and some bilingual effects on speaking increase as bilinguals age. An obvious (and popular) account of the bilingual effect on speaking is that knowing two words to express any given concept introduces competition between languages. However, we propose that differences in patterns of language use alone are sufficient to explain some of the consequences of bilingualism for speaking. We investigate bilingual effects on speaking with the goal of constraining models of bilingualism and of language production in all speakers (Section I). We hypothesize that bilinguals differ from monolinguals in circumscribed ways that are either related to their different patterns of language use or arise because the same tasks sometimes present bilinguals and monolinguals with different challenges. We test the predictions of two different accounts of how bilingualism should affect single word production, sentence level fluency, and speech errors (including tip-of-the-tongue or TOT states, whole word substitutions, and sound errors). We also aim to develop models of bilingual control and to identify processing mechanisms that may be unique to bilinguals by introducing a previously uninvestigated type of language switching and by explicitly manipulating the degree of dual-language activation (Section II). The mechanisms we consider make different predictions about how aging should affect bilingual performance, and we test these predictions as a means of developing models of bilingualism and accounts of age-related changes in cognitive performance. By identifying the generality of the bilingual effect, we determine what constitutes normal performance for young and aging bilinguals in a variety of tasks, and will identify the conditions that allow bilinguals of different types to function as fluently as they can. The proposed experiments will constrain models of monolingual and bilingual language production, models of bilingual and cognitive control, and accounts of age-related processing changes including both processing disadvantages (e.g., age-related decline in cognitive control) but also processing advantages (e.g., older adults have increased experience with words).